The Field Barrels were only made in one length I believe which was either the 5" or 5.5" model. Plus they were a older design as well which means it should be the real good dark Blue stain on them also.
There was a story on here about the making of the Field Barrels years ago. They were slightly damaged longer barrels just cut down so they could be re-used as they may have been slightly defective in much longer lengths. To me it was Good use of scrap parts otherwise.